July/August 2008

I live in a 5-mile-wide, north-south river valley about 30 miles east of Madison, Wisconsin. The valley is only about 100 feet lower than the surrounding rolling countryside, yet I often record less precipitation than cooperative observers nearby who do not live in the valley. I am in Wisconsin farm country, without urban or forest effects. There are no mountains or large lakes nearby, just rolling hills. The effects I notice are: 1) In the early stages of large stratiform events, when the leading edge of precipitation generally must fall through dry air, it takes longer for the column to saturate at my place than the nearby higher ground. This is especially noticeable during snowfalls, when there are few flakes falling at my place, yet the higher lands are obscured by snow. Sometimes I miss at least an inch of snow that way. 2) Often the heaviest parts of localized convective showers die out in our valley and regenerate once they get to higher ground. I assume this drying is from a net sinking effect, depending on whether the winds come off the higher ground. If the winds are more parallel with the river valley, the effect is less noticeable. What amount of elevation change is generally needed to create an annual rain and snowfall difference of at least one inch between two precipitation gauges located within 10 miles of each other, all other terrain effects being equal?

--Dave NolanLake Mills, Wisconsin

I have two grown children living near the Green Bay. During one winter visit to the area I personally witnessed one of the effects you mentioned. Northwest winds ascending low ridges perhaps 100-150 feet high southeast of Green Bay delivered 2 to 3 inches of snow on the ridges but only about 1 inch in the nearby Fox River Valley during a 12-hour period.

The answer to your question is not straightforward because of two influences on Wisconsin’s annual precipitation beyond topography. The first is annual mean temperature, which ranges from 39°F in the north to 50°F in the south. All other things being equal, one expects higher precipitation where the climate is warmer, because the maximum amount of water vapor that can exist in air is strongly limited by temperature: less when the air is cold, more when it is warm. Figure 1, a color-coded map of mean annual precipitation, confirms this tendency.